Each has their audience. Unreal is going for that "extreme graphics" brand, while Unity is going for more for a broader audience and not focusing so much on graphics. Both of them are companies (like any other company), and need users. Each of them are trying to knock out a part of the market.
If you want to make production-level games with Allegro, you will have to put in some work. Like Dizzy said, you'll need to build a shader pipeline, add loader code for models, create animation and rigging systems, and consider the ramifications of cross-platform quirks.
Allegro does provide a good foundation for you to do these things, specifically in cross-platform support. The alternative is that you use the OS's APIs directly, but who wants to do that when Allegro already has such a nice abstraction API. So Allegro has a place.
If you want to make games, you'll need to have tools like level editors, 3D modeling software, pixel art tools, etc. These things are not provided by Allegro, but other game development tools do include things with this purpose. The typical path that Allegro users have taken is to spend time building out their own level editors. Realistically, this might not be the best use of development time.
I personally prefer to build out my own workflows, so I love things like Allegro.
Any other tool that provides APIs or has scripting support (Tiled, Blender, FLStudio, etc) is just incredibly powerful (did you know you can run Blender headless?